Most tanks take a role of responsibility in most runs they participate in. They mark targets, explain strategy, and are often times praised or blamed for the way a run turns out.
Tanking is a big responsibility in-game. Some people do not tank for this very reason. It is much easier to manage a damage rotation and watch pretty numbers fly. DPS may be somewhat complicated (like using a proper rotation), but they dont generally have to worry about other things.
Nobody else usually worries about that stray mob running off towards the healer. It is the tanks job to charge off and taunt it. Sure you get the rare hunter from time to time who will throw and ice trap at the healer's feet, but most DPS dont care to stop pew pewing.
Tanks are the ones who everyone expects to know the boss fight and mark trash mobs for kill orders. They also expect each symbol to be explained. This is not always the case as it may be a tanks first time in. Tanks are the ones who are expected to do the research and do their job right the first time.
With all of this pressure, it is no wonder that there are tank shortages at times. Some people just dont get it. Those of us that do, have tanking in our blood. We love being the meat shield that keeps everyone safe. Healers feel much of the same pains as us tanks. They are expected to keep everyone up despite whatever mistakes are made.
One important lesson for all tanks. If you dont know, then just ask. They may expect you to know it, but if you dont, speak up. Part of being a tank is learning and adapting. Dont be afraid to speak up. Despite the AOE fest that is Wrath, don't be afraid to ask for some CC. Pretty much every class has some form of crowd control they can do.
Everyone want a tank that makes their job easy. Dont be afraid to ask for and expect the same in return.